Thin-film transistor (TFT) devices are widely used in switching or driver circuitry for electro-optical arrays and display panels. TFT devices are conventionally fabricated on rigid substrates, typically glass or silicon, using a well-known sequence of deposition, patterning and etching steps. For example, amorphous silicon TFT devices require deposition, patterning, and etching of metals, such as aluminum, chromium or molybdenum, of amorphous silicon semiconductors, and of insulators, such as SiO2 or Si3N4 onto a substrate. The semiconductor thin film is formed in layers having typical thicknesses of from several nm to several hundred nm, with intermediary layers having thicknesses on the order of a few microns, and may be formed over an insulating surface that lies atop the rigid substrate.
The requirement for a rigid substrate is based largely on the demands of the fabrication process itself. Thermal characteristics are of particular importance, since TFT devices are fabricated at relatively high temperatures. Thus, the range of substrate materials that have been used successfully is somewhat limited, generally to glass, quartz, or other rigid, silicon-based materials. Among other problems, the limitations inherent to these substrates frustrate efforts to fabricate larger displays and devices, since crystalline silicon and many types of glass that are conventionally employed as substrates become increasingly difficult to form and manage in large, thin, lightweight sheets.
Clearly, there would be advantages in expanding the range of suitable substrate materials for the formation of electronic devices. As part of the effort to extend the range of possible substrates, TFT devices have been formed on some types of metal foil substrates. Metal substrates are advantaged for reduced weight, efficient thermal dissipation, and overall robustness when exposed to mechanical stress, flexing, or shock. It has been recognized that their overall thermal, dimensional, and chemical stability makes metal foil substrates a desirable substrate for electronic devices such as active matrix organic light-emitting diode (AMOLED) transistor backplanes that are conventionally formed on glass or silicon. However, problems with surface morphology, resulting in increased capacitive coupling, edge effects, and possible shorting between layered components formed thereon, make metal foil substrates difficult to fabricate for use with AMOLED or other thin-film devices. Thus, while they exhibit many desirable properties and offer significant potential for lightweight component designs, metal substrates have yet to be shown as practical replacements for glass or silicon wafer materials.
One major hurdle with conventional approaches to providing and preparing a metal substrate suitable for thin-film device fabrication thereon relates to surface characteristics using existing surface treatment techniques. High quality stainless steel, for example, can be roll-formed to provide a surface that is smooth to within about 1.4 to 2 microns peak-to-peak. However, for thin-film devices, smoothness of at least no more than 400 nm, more preferably no more than about 200-300 nm peak-to-peak is desirable. Any rougher surface would require an excessively thick planarization layer, more likely subject to cracking and other defects. While there are adaptable wafer-surface treatment techniques for smoothing the surface, such as chemical-mechanical polishing (CMP), such techniques are very costly and are limited in treatment surface area. Another difficulty with conventional techniques are relatively high defect rates.
For these reasons, the potential advantages metal substrates would offer for thin-film device formation have yet to be realized. It can, therefore, be appreciated that there is a need for an economical method for forming a metal substrate with a surface smoothness that allows fabrication of TFT electronic devices thereon.